Extreme ultraviolet light reflective structure including nano-lattice and manufacturing method thereof

ABSTRACT

An EUV reflective structure includes a substrate and multiple pairs of a Si layer and a Mo layer. The Si layer includes a plurality of cavities.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/753,913 filed Oct. 31, 2018, the entire contents of which isincorporated herein by reference.

BACKGROUND

The wavelength of radiation used for lithography in semiconductormanufacturing has decreased from ultraviolet to deep ultraviolet (DUV)and, more recently to extreme ultraviolet (EUV). Further decreases incomponent size require further improvements in resolution of lithographywhich are achievable using extreme ultraviolet lithography (EUVL). EUVLemploys radiation having a wavelength of about 1-100 nm, e.g., 13.5 nm.Since a projection lens type exposure apparatus cannot be used in an EUVlithography, all reflective optical system is required in the EUVlithography. Accordingly, an EUV reflective structure (reflector, suchas a mirror) having a high reflectance is one of the key technology inthe EUV lithography.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a schematic view of an EUV lithography system with a laserproduced plasma (LPP) EUV radiation source in accordance with someembodiments of the present disclosure.

FIG. 2 shows a schematic diagram of an extreme ultraviolet lithographytool according to an embodiment of the disclosure.

FIG. 3A is a perspective view of an EUV reflective structure accordingto an embodiment of the present disclosure. FIG. 3B is a plan view ofcavities when horizontally cutting the Si layer, FIG. 3C is a crosssectional view and FIG. 3D is an enlarged perspective view of a unitstructure of the EUV reflective structure shown in FIG. 3A.

FIG. 4A is a side view of a cavity and FIG. 4B is a plan view of thecavity according to an embodiment of the disclosure. FIG. 4C is a planview of the cavities according to another embodiment of the disclosure.FIG. 4D is a plan view of the cavities according to another embodimentof the disclosure.

FIG. 5A is a perspective view of an EUV reflective structure accordingto an embodiment of the present disclosure. FIG. 5B is a plan view ofthe cavities, FIG. 5C is a cross sectional view and FIG. 5D is anenlarged perspective view of a unit structure of the EUV reflectivestructure shown in FIG. 5A.

FIG. 6A is a perspective view of an EUV reflective structure accordingto an embodiment of the present disclosure. FIG. 6B is a plan view ofcavities, FIG. 6C is a cross sectional view and FIG. 6D is an enlargedperspective view of a unit structure of the EUV reflective structureshown in FIG. 6A. FIG. 6E shows an arrangement of Si pillars accordingto an embodiment of the disclosure. FIG. 6F shows an arrangement of Sipillars according to another embodiment of the disclosure.

FIGS. 7A, 7B and 7C show various cross sectional views of EUV reflectivestructures according to embodiments of the disclosure.

FIG. 8 is a cross sectional view of an EUV reflective mirror accordingto an embodiment of the disclosure.

FIG. 9 shows simulation results of reflectivity for the EUV reflectivestructures according to embodiments of the disclosure.

FIG. 10 shows an EUV photo mask according to an embodiment of thedisclosure.

FIG. 11 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 12 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 13 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 14 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 15 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 16 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 17 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 18 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 19 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIGS. 20A, 20B, 20C and 20D show views of the various stages of asequential manufacturing operation for an EUV reflective structureaccording to an embodiment of the present disclosure.

FIG. 21 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 22 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 23 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 24 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 25 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

FIG. 26 shows a view of one of the various stages of a sequentialmanufacturing operation for an EUV reflective structure according to anembodiment of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“being made of” may mean either “comprising” or “consisting of.” In thepresent disclosure, a phrase “one of A, B and C” means “A, B and/or C”(A, B, C, A and B, A and C, B and C, or A, B and C), and does not meanone element from A, one element from B and one element from C, unlessotherwise described.

The present disclosure is generally related to an extreme ultraviolet(EUV) reflector, such as a EUV reflective mirror and a EUV photo mask.In the following embodiments, materials, configuration, dimensions,processes and/or method explained with respect to one embodiments can beapplied to other embodiments, and the detailed description thereof maybe omitted.

FIG. 1 is a schematic and diagrammatic view of an EUV lithographysystem. The EUV lithography system includes an EUV radiation sourceapparatus 100 to generate EUV light, an exposure tool 200, such as ascanner, and an excitation laser source apparatus 300. As shown in FIG.1, in some embodiments, the EUV radiation source apparatus 100 and theexposure tool 200 are installed on a main floor MF of a clean room,while the excitation source apparatus 300 is installed in a base floorBF located under the main floor. Each of the EUV radiation sourceapparatus 100 and the exposure tool 200 are placed over pedestal platesPP1 and PP2 via dampers DP1 and DP2, respectively. The EUV radiationsource apparatus 100 and the exposure tool 200 are coupled to each otherby a coupling mechanism, which may include a focusing unit.

The lithography system is an extreme ultraviolet (EUV) lithographysystem designed to expose a resist layer by EUV light (or EUVradiation). The resist layer is a material sensitive to the EUV light.The EUV lithography system employs the EUV radiation source apparatus100 to generate EUV light, such as EUV light having a wavelength rangingbetween about 1 nm and about 100 nm. In one particular example, the EUVradiation source 100 generates an EUV light with a wavelength centeredat about 13.5 nm. In the present embodiment, the EUV radiation source100 utilizes a mechanism of laser-produced plasma (LPP) to generate theEUV radiation.

The exposure tool 200 includes various reflective optic components, suchas convex/concave/flat mirrors, a mask holding mechanism including amask stage, and wafer holding mechanism. The EUV radiation EUV generatedby the EUV radiation source 100 is guided by the reflective opticalcomponents onto a mask secured on the mask stage. In some embodiments,the mask stage includes an electrostatic chuck (e-chuck) to secure themask. Because gas molecules absorb EUV light, the lithography system forthe EUV lithography patterning is maintained in a vacuum or a-lowpressure environment to avoid EUV intensity loss.

FIG. 2 is a simplified schematic diagram of a detail of an extremeultraviolet lithography tool according to an embodiment of thedisclosure showing the exposure of photoresist coated substrate 210 witha patterned beam of EUV light. The exposure device 200 is an integratedcircuit lithography tool such as a stepper, scanner, step and scansystem, direct write system, device using a contact and/or proximitymask, etc., provided with one or more optics 205 a, 205 b, for example,to illuminate a patterning optic 205 c, such as a reticle, with a beamof EUV light, to produce a patterned beam, and one or more reductionprojection optics 205 d, 205 e, for projecting the patterned beam ontothe substrate 210. A mechanical assembly (not shown) may be provided forgenerating a controlled relative movement between the substrate 210 andpatterning optic 205 c. As further shown in FIG. 2, the EUVL toolincludes an EUV light source 100 including plasma at ZE emitting EUVlight in a chamber 105 that is collected and reflected by a collector110 along a path into the exposure device 200 to irradiate the substrate210.

As used herein, the term “optic” is meant to be broadly construed toinclude, and not necessarily be limited to, one or more components whichreflect and/or transmit and/or operate on incident light, and includes,but is not limited to, one or more lenses, windows, filters, wedges,prisms, grisms, gratings, transmission fibers, etalons, diffusers,homogenizers, detectors and other instrument components, apertures,axicons and mirrors including multi-layer mirrors, near-normal incidencemirrors, grazing incidence mirrors, specular reflectors, diffusereflectors and combinations thereof. Moreover, unless otherwisespecified, the term “optic,” as used herein, is not meant to be limitedto components which operate solely within one or more specificwavelength range(s) such as at the EUV output light wavelength, theirradiation laser wavelength, a wavelength suitable for metrology or anyother specific wavelength.

In the present disclosure, the terms mask, photomask, and reticle areused interchangeably. In the present embodiment, the mask is areflective mask. One exemplary structure of the mask includes asubstrate with a suitable material, such as a low thermal expansionmaterial or fused quartz. In various examples, the material includesTiO₂ doped SiO₂, or other suitable materials with low thermal expansion.The mask includes multiple reflective multiple layers deposited on thesubstrate. The multiple layers include a plurality of film pairs, suchas molybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenumabove or below a layer of silicon in each film pair). Alternatively, themultiple layers may include molybdenum-beryllium (Mo/Be) film pairs, orother suitable materials that are configurable to highly reflect the EUVlight. The mask may further include a capping layer, such as ruthenium(Ru), disposed on the ML for protection. The mask further includes anabsorption layer, such as a tantalum boron nitride (TaBN) layer,deposited over the multiple layers. The absorption layer is patterned todefine a layer of an integrated circuit (IC). Alternatively, anotherreflective layer may be deposited over the multiple layers and ispatterned to define a layer of an integrated circuit, thereby forming anEUV phase shift mask.

In the present embodiments, the semiconductor substrate is asemiconductor wafer, such as a silicon wafer or other type of wafer tobe patterned. The semiconductor substrate is coated with a resist layersensitive to the EUV light in the present embodiment. Various componentsincluding those described above are integrated together and are operableto perform lithography exposing processes. The lithography system mayfurther include other modules or be integrated with (or be coupled with)other modules.

As shown in FIG. 1, the EUV radiation source 100 includes a targetdroplet generator 115 and a LPP collector 110, enclosed by a chamber105. The target droplet generator 115 generates a plurality of targetdroplets DP. In some embodiments, the target droplets DP are tin (Sn)droplets. In some embodiments, the tin droplets each have a diameterabout 30 microns (am). In some embodiments, the tin droplets DP aregenerated at a rate about 50-50000 droplets per second and areintroduced into a zone of excitation ZE at a speed about 70 meters persecond (m/s). Other material can also be used for the target droplets,for example, a tin containing liquid material such as eutectic alloycontaining tin or lithium (Li).

The excitation laser LR2 generated by the excitation laser sourceapparatus 300 is a pulse laser. In some embodiments, the excitationlaser includes a pre-heat laser and a main laser. The pre-heat laserpulse is used to heat (or pre-heat) the target droplet to create alow-density target in a pancake shape, which is subsequently heated (orreheated) by the main laser pulse, generating increased emission of EUVlight. In various embodiments, the pre-heat laser pulses have a spotsize about 100 am or less, and the main laser pulses have a spot sizeabout 200-300 am.

The laser pulses LR2 are generated by the excitation laser source 300.The laser source 300 may include a laser generator 310, laser guideoptics 320 and a focusing apparatus 330. In some embodiments, the lasersource 310 includes a carbon dioxide (CO₂) or a neodymium-doped yttriumaluminum garnet (Nd:YAG) laser source. The laser light LR1 generated bythe laser generator 300 is guided by the laser guide optics 320 andfocused into the excitation laser LR2 by the focusing apparatus 330, andthen introduced into the EUV radiation source 100.

The laser light LR2 is directed through windows (or lenses) into thezone of excitation ZE. The windows adopt a suitable materialsubstantially transparent to the laser beams. The generation of thepulse lasers is synchronized with the generation of the target droplets.As the target droplets move through the excitation zone, the pre-pulsesheat the target droplets and transform them into low-density target in apancake shape. A delay between the pre-pulse and the main pulse iscontrolled to allow the target in a pancake shape to form and to expandto an optimal size and geometry. When the main pulse heats the target ina pancake shape, a high-temperature plasma is generated. The plasmaemits EUV radiation EUV, which is collected by the collector mirror 110.The collector 110 has a reflection surface that reflects and focuses theEUV radiation for the lithography exposing processes. In someembodiments, a droplet catcher 116 is installed opposite the targetdroplet generator 115. The droplet catcher 116 is used for catchingexcess target droplets. For example, some target droplets may bepurposely missed by the laser pulses.

The collector 110 includes a proper coating material and shape tofunction as a mirror for EUV collection, reflection, and focusing. Insome embodiments, the collector 110 is designed to have an ellipsoidalgeometry. In some embodiments, the coating material of the collector 100is similar to the reflective multilayer of the EUV mask. In someexamples, the coating material of the collector 110 includes multiplelayers (such as a plurality of Mo/Si film pairs) and may further includea capping layer (such as Ru) coated on the multiple layers tosubstantially reflect the EUV light. In some embodiments, the collector110 may further include a grating structure designed to effectivelyscatter the laser beam directed onto the collector 110. For example, asilicon nitride layer is coated on the collector 110 and is patterned tohave a grating pattern in some embodiments.

In such an EUV radiation source apparatus, the plasma caused by thelaser application creates physical debris, such as ions, gases and atomsof the droplet, as well as the desired EUV radiation. It is necessary toprevent the accumulation of material on the collector 110 and also toprevent physical debris exiting the chamber 105 and entering theexposure tool 200.

As shown in FIG. 1, in some embodiments, a buffer gas is supplied from afirst buffer gas supply 130 through the aperture in collector 110 bywhich the pulse laser is delivered to the tin droplets. In someembodiments, the buffer gas is H₂, He, Ar, N₂, or another inert gas. Incertain embodiments, H₂ is used as H radicals generated by ionization ofthe buffer gas can be used for cleaning purposes. The buffer gas canalso be provided through one or more second buffer gas supplies 135toward the collector 110 and/or around the edges of the collector 110.Further, the chamber 105 includes one or more gas outlets 140 so thatthe buffer gas is exhausted outside the chamber 105. Hydrogen gas haslow absorption to the EUV radiation. Hydrogen gas reaching the coatingsurface of the collector 110 reacts chemically with a metal of thedroplet forming a hydride, e.g., metal hydride. When tin (Sn) is used asthe droplet, stannane (SnH₄), which is a gaseous byproduct of the EUVgeneration process, is formed. The gaseous SnH₄ is then pumped outthrough the outlet 140. However, it is difficult to exhaust all gaseousSnH₄ from the chamber and to prevent the SnH₄ from entering the exposuretool 200. To trap the SnH₄ or other debris, one or more debriscollection mechanisms or devices 150 are employed in the chamber 105.

As shown in FIG. 2, the EUV lithography system requires various EUVreflective structures, such as flat and curved mirrors. Many of the EUVreflective structures include multiple reflective layers (ML) depositedon a substrate, as an EUV reflective structure. The ML includes aplurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs(e.g., a layer of molybdenum above or below a layer of silicon in eachfilm pair). In the present disclosure, nano cavities 25 are periodicallyarranged in the Si layer of the ML system to increase EUV reflectivity.

Multilayers are complicated systems and substantially impossible todescribe analytically. In general, one can solve the Maxwell equationnumerically with certain boundary conditions, since they are physicallythe fundamental of electro-magnetic waves (light) and mathematically aset of partial differential equations:

${{\overset{arrow}{\nabla}{\cdot \overset{arrow}{D}}} = 0},{{\overset{arrow}{\nabla}{\times \overset{arrow}{E}}} = \frac{\partial\overset{arrow}{B}}{\partial t}},{{\overset{arrow}{\nabla}{\cdot \overset{arrow}{B}}} = 0},{{\overset{arrow}{\nabla}{\times \overset{arrow}{H}}} = {\frac{\partial\overset{arrow}{D}}{\partial t} + \overset{arrow}{J}}}$

Only simple layer problems can be completely understood analytically.

Transmitted light travels longer distance due to refraction. This causea phase difference when the transmitted light reflects back to the top,which can lead to interference effects. This can be used to makeanti-reflection coatings.

The incident EUV light becomes a standing-wave when entering the Mo/Simultilayer. Mo has a greater absorption coefficient than Si:

β_(Mo)>β_(si),Δβ=β_(Mo)−β_(Si),

thus it is used for the low-intensity standing-wave field. Placing nanocavities into the Si-layer causes a decrease in the absorptioncoefficient, which enhances resonance and therefore the total reflectedintensity (maximize Δβ).

FIG. 3A is a perspective view of an EUV reflective structure accordingto an embodiment of the present disclosure. FIG. 3B is a plan view ofthe cavities when horizontally cutting the Si layer, FIG. 3C is a crosssectional view and FIG. 3D is an enlarged perspective view of a unitstructure of the EUV reflective structure shown in FIG. 3A.

As shown in FIGS. 3A-3D, the EUV reflective structure includes firstlayers 20 and second layers 30 alternately stacked. In some embodiments,the first layers 20 are made of Si and the second layers 30 are made ofMo.

In some embodiments, nano cavities 25 are arranged in each Si layer 20of the whole Mo/Si-multilayer system 15. In some embodiments, thenano-cavities 25 have a cubic shape, a polygonal pillar shape and/or acylindrical shape. In certain embodiments, the nano cavities 25 areperiodically arranged in the Si layer 20. In some embodiments, the nanocavities 25 are periodically arranged within one Si layer (in the X-Yplane). In other embodiments, the nano cavities 25 are periodicallyarranged in the all directions. In other embodiments, at least one ofthe Si layers 20 includes the nano cavities 25, and at least one of theSi layers 20 includes no nano cavities 25.

In some embodiments, the nano cavities 25 are filled with a gas, such asair or inert gas (Ar, He, Ne and/or N₂, etc). In some embodiments, thegas pressure is atmospheric. In other embodiments, the nano cavities 25are maintained at a pressure lower than 1 Pa (vacuum). In certainembodiments, the nano cavities 25 are filled with a dielectric material,such as silicon oxide.

As shown in FIGS. 3A-3D, the nano cavities 25 are fully embedded(sealed) in the Si layer in some embodiments. In other words, each ofthe nano cavities 25 is fully surrounded by a Si body of the Si layer20.

In some embodiments, each of the plurality of cavities has an area inplan view (in X-Y plane) in a range from about 1 nm² to 10000 nm². Inother embodiments, the area in plan view of the cavity is in a rangefrom about 10 nm² to 1000 nm². In some embodiments, a volume of each ofthe plurality of cavities is in a range from about 1 nm³ to 1,000,000nm³. In other embodiments, the volume of the cavity is in a range fromabout 30 nm³ to 30,000 nm³.

In some embodiments, each of the nano cavities 25 has a polygonal (e.g.,hexagonal) pillar shape. In certain embodiments, each of the nanocavities 25 has a cubic shape. In other embodiments, each of theplurality of cavities has a cylindrical pillar shape as shown in FIGS.4A and 4B. In some embodiments, the nano cavities 25 are arranged in asimple matrix as shown in FIG. 3B. In some embodiments, as shown in FIG.4C, the nano cavities 25 have a honeycomb arrangement (hexagonalbottom/top shape). In other embodiments, as shown in FIG. 4D, the nanocavities 25 are arranged in a diagonal-orthogonal arrangement. Thesearrangements can increase thermal stability and prevent cracking.

As shown in FIGS. 3B, 3C and 3D, the nano cavities 25 are arranged in amatrix in the Si layer 20 with a constant pitch P. In some embodiments,the pitch in the X direction is equal to the pitch in the Y direction.In some embodiments, the pitch P is in a range from about 5 nm to about50 nm and is in a range from about 10 nm to about 25 nm in otherembodiments.

In other embodiments, the pitches in the X and Y directions aredifferent from each other. In some embodiments, a thickness of eachSi/Mo pair 16 is in a range from 6 nm to 8 nm, and is in a range from6.5 nm to 7.5 nm in other embodiments. In some embodiments, a thicknessTs of the Si layer 20 and a thickness Tm of the Mo layer 30 satisfyTs:Tm=5:5 to 7:3. In some embodiments, the total number of the Si/Mopairs 16 is at least 40. In some embodiments, the total number of theSi/Mo pairs 16 is less than 100. In some embodiments, the thickness ofthe Si layer under the cavities is equal to the thickness of the Silayer above the cavities. In other embodiments, the thickness of the Silayer under the cavities is different from the thickness of the Si layerabove the cavities.

In some embodiments, no cavity is disposed in the Mo layer. In someembodiments, the Mo layer 30 is disposed on the Si layer 20. In otherwords, the Si layer 20 is in contact with a substrate 10 and the Molayer 30 is the uppermost layer of the multilayer structure 15. Thesubstrate 10 is a suitable material, such as a low thermal expansionmaterial or fused quartz. In various examples, the material of thesubstrate 10 includes TiO₂ doped SiO₂, or other suitable materials withlow thermal expansion. In some embodiments, one or more layers areinserted between the substrate 10 and the lowest Si layer 20.

In some embodiments, the Si/Mo multilayer structure 15 can be fabricatedby one or more film deposition, lithography and etching operations. Insome embodiments, a Si layer is formed over the underlying layer, e.g.,substrate 10 by using chemical vapor deposition (CVD), physical vapordeposition (PVD) including sputtering, atomic layer deposition (ALD) orany other suitable film formation techniques. Then, by using one or morelithography and etching operations, holes are formed in the Si layer.Next, another Si layer is formed to cover the holes, thereby formingnano cavities. In some embodiments, before another Si layer is formed,the nano cavities are filled with dielectric material or a gas. Then, aMo layer is formed by CVD, PVD, ALD, or any other suitable filmformation techniques. In some embodiments, a thin Si layer is separatelyformed, and transferred over the Si layer with the holes. In certainembodiments, a bilayer of Si and Mo is separately formed, andtransferred over the Si layer with the holes.

FIG. 5A is a perspective view of an EUV reflective structure accordingto another embodiment of the present disclosure. FIG. 5B is a plan viewof the nano cavities 25, FIG. 5C is a cross sectional view and FIG. 5Dis an enlarged perspective view of a unit structure of the EUVreflective structure shown in FIG. 5A. Material, configurations,dimensions, processes and/or methods as described with respect to theabove embodiments may be employed in the following embodiments, anddetailed explanation thereof may be omitted.

In the embodiment of FIGS. 5A-5D, the top face of each of the nanocavities 25 is covered by the Mo layer 30. Thus, the nano cavities 26are sealed by the body of the Si layer 20 at the bottom and sides and bythe Mo layer 30 at the top.

In some embodiments, the Si/Mo multilayer structure 15 shown in FIGS.5A-5D can be fabricated by one or more film deposition, lithography andetching operations. In some embodiments, a Si layer is formed over theunderlying layer, e.g., substrate 10 by using CVD, PVD, ALD or any othersuitable film formation techniques. Then, by using one or morelithography and etching operations, holes are formed in the Si layer.Next, a Mo layer is formed by CVD, PVD, ALD, or any other suitable filmformation techniques. In some embodiments, before the Mo layer isformed, the nano cavities are filled with dielectric material or a gas.In some embodiments, a Mo layer is separately formed, and transferredover the Si layer with the holes.

FIG. 6A is a perspective view of an EUV reflective structure accordingto an embodiment of the present disclosure. FIG. 6B is a plan view ofthe nano cavities 25, FIG. 6C is a cross sectional view and FIG. 6D isan enlarged perspective view of a unit structure of the EUV reflectivestructure shown in FIG. 6A. Material, configurations, dimensions,processes and/or methods as described with respect to the aboveembodiments may be employed in the following embodiments, and detailedexplanation thereof may be omitted.

In this embodiment, the Si layers 20 are constituted by a plurality ofSi pillars 22. The nano cavities 27 are areas among the pillars 22, andsealed by the Mo layers at the top and the bottom. The width of each ofthe Si pillars is in a range from about 3 nm to about 25 nm in someembodiments, and is in a range from about 5 nm to about 15 nm in otherembodiments.

In some embodiments, the Si/Mo multilayer structure 15 shown in FIGS.6A-6D can be fabricated by one or more film deposition, lithography andetching operations. In some embodiments, a Si layer is formed over theunderlying layer, e.g., substrate 10 by using CVD, PVD, ALD or any othersuitable film formation techniques. Then, by using one or morelithography and etching operations, the Si pillars are formed. Next, aMo layer is formed by CVD, PVD, ALD, or any other suitable filmformation techniques. In some embodiments, a Mo layer is separatelyformed, and transferred over the Si pillars.

In some embodiments, after the Si pillars are formed, dielectricmaterial, such as silicon oxide or silicon nitride, is formed to fillspaces between the Si pillars. Then, a planarization operation, such aschemical mechanical polishing (CMP), is performed to expose the topfaces of the Si pillars. Subsequently, a Mo layer is formed. In someembodiments, after the Mo layer is formed, the dielectric material isremoved by using a wet etching technique.

FIG. 6E shows an arrangement of the Si pillars 22 according to anembodiment of the disclosure. In FIG. 6E, the Si pillars 22 are arrangedin a simple matrix. FIG. 6F shows another arrangement of Si the pillars22 according to another embodiment of the disclosure. In FIG. 6F, the Sipillars 22 are arranged in a hexagonal matrix.

FIGS. 7A, 7B and 7C show various cross sectional views of EUV reflectivestructures according to embodiments of the disclosure. Material,configurations, dimensions, processes and/or methods as described withrespect to the above embodiments may be employed in the followingembodiments, and detailed explanation thereof may be omitted.

In the embodiments of FIGS. 7A-7C, one or more cap layers are disposedover the multilayer structure 15.

In some embodiments, as shown in FIG. 7A, the cap layer is a protectivelayer 52 made of a material that can prevent the multilayer stack 15from oxidation. In some embodiments, the protective layer 52 is asilicon nitride layer or a polymer layer.

In some embodiments, as shown in FIG. 7B, the cap layer is an absorberlayer 54 made of a material that can absorb the EUV light. In someembodiments, the absorber layer 54 is a TaBN layer. In some embodiments,the TaBN layer includes a circuit pattern, and thus the EUV reflectivestructure is used as an EUV photo mask.

In some embodiments, as shown in FIG. 7C, the cap layer is a gratinglayer 56 having a refractive structure. In some embodiments, therefractive structure 56 is made of silicon oxide, silicon nitride, Mo,Zr, Ti, Ta, W or any other suitable material.

FIG. 8 is a cross sectional view of an EUV reflective mirror accordingto an embodiment of the disclosure. Material, configurations,dimensions, processes and/or methods as described with respect to theabove embodiments may be employed in the following embodiments, anddetailed explanation thereof may be omitted

As shown in FIG. 8, the substrate 10 has a curved surface having adesired curvature or reflective properties (focus position, etc) in atleast the upper surface thereof. The Si/Mo multilayer structure 15 isformed on the curved surface.

FIG. 9 shows simulation results of reflectivity for the EUV reflectivestructures according to embodiments of the disclosure. The graph showsthe EUV reflectivity with respect to the volume of the cavity in a unitstructure. The unit structure has a dimension Px (a size in the Xdirection), Py (a size in the Y direction) and d_(Si) (the thickness ofthe Si layer). In some embodiments, Px and Py correspond to the pitchesof the nano cavities 25 in both directions. Further, “a”, “b and “c” aredimensions of the nano cavity. When the nano cavity and the unitstructure are cubic, the volume of the nano cavity is a³ and the volumeof the unit structure is d_(Si) ³. In some embodiments, a, b and c arein a range from about 1 nm to about 50 nm, respectively, and are in arange from about 2 nm to about 10 nm.

As shown in FIG. 9, when no cavities are disposed, the EUV reflectivityof about 70% at 13.5 nm light. When nano cavities 25 are introduced, theEUV reflectivity is more than 73%. With the cavity aspect ratio (a ratioof the cavity volume to the volume of unit structure defined by thepitches and thickness of the Si layer) increases, the EUV reflectivityincreases. In some embodiments, the EUV reflectivity is more than 80%,when the cavity aspect ratio is 0.7 (70%) or more. In some embodiments,the ratio is in a range from about 0.3 to 0.9. In certain embodiments,when the plurality of cavities are arranged in a matrix with a pitch p1in one direction and a pitch p2 in another direction crossing the onedirection, and the Si layer has a thickness t, a volume Vn of each ofthe plurality of cavities satisfies 0.3≤Vn/(t×p1×p2)≤0.9.

FIG. 10 shows an EUV photo mask according to an embodiment of thedisclosure. In the present disclosure, the terms mask, photomask, andreticle are used interchangeably. In the present embodiment, thepatterning optic shown in FIG. 10 is a reflective reticle. In anembodiment, the reflective reticle includes a substrate 510 formed of asuitable material, such as a low thermal expansion material or fusedquartz, similar to the substrate 10. In various examples, the materialof the substrate 510 includes TiO₂ doped SiO₂, or other suitablematerials with low thermal expansion. The reflective reticle includesmultiple reflective layers (ML) 515 according to the embodiments as setforth above. The reticle may further include a capping layer 540, suchas ruthenium (Ru), disposed on the ML for protection. The mask furtherincludes an absorption layer 545, such as a tantalum boron nitride(TaBN) layer, deposited over the ML 515. The absorption layer 545 ispatterned to define a layer of an integrated circuit (IC). Thereflective reticle includes a conductive backside coating 560.Alternatively, another reflective layer may be deposited over the ML 515and is patterned to define a layer of an integrated circuit, therebyforming an EUV phase shift reticle.

FIGS. 11-19 shows views of various stages of a sequential manufacturingoperation for an EUV reflective structure according to an embodiment ofthe present disclosure. It is understood that additional operations canbe provided before, during, and after processes shown by FIGS. 11-19,and some of the operations described below can be replaced oreliminated, for additional embodiments of the method. The order of theoperations/processes may be interchangeable. Materials, configuration,dimensions, processes and/or method explained with respect to the aboveembodiments can be applied to the following embodiments, and thedetailed description thereof may be omitted.

As shown in FIG. 11, a Si layer 620 is formed over a substrate 610, forexample a quartz substrate. Then, as shown in FIG. 12, a mask pattern630 is formed over the Si layer 620. In some embodiments, the maskpattern 630 is a photo resist pattern. In other embodiments, the maskpattern 630 is a hard mask pattern made of, for example, silicon oxideand/or silicon nitride. As shown in FIG. 13, the Si layer 620 is thenpatterned by using the mask pattern 630 as an etching mask. In someembodiments, the patterned Si (Si patterns) 625 are Si pillars. In otherembodiments, the patterned Si layer is a framed shaped.

Next, as shown in FIG. 14, a sacrificial layer 640 is form over the Sipatterns 625. In some embodiments, the sacrificial layer 640 includesone or more of silicon oxide, silicon nitride, SiON, SiOC, SiOCN or anyother suitable material that can be selectively removed with respect toSi and Mo. The sacrificial layer 640 is formed by ALD or CVD in someembodiments. Then, as shown in FIG. 15, a planarization operation, suchas CMP, is performed to expose the Si patterns 625. In some embodiments,the height of the Si patterns 625 is adjusted (reduced) by the CMPoperation. A Mo layer 650 is then formed on the Si patterns 625 and thesacrificial layer 640 as shown in FIG. 16. The Mo layer 650 is formed byPVD, CVD or ALD in some embodiments. Further, an additional Si layer 660is formed on the Mo layer 650 as shown in FIG. 17. The operations offorming a Si layer as shown in FIG. 15 to forming an additional Si layeras shown in FIG. 17 are repeated for about 30-50 times, and thus thestructure as shown in FIG. 18 is obtained. Note that, repeats of thisprocesses more than 50 times is also possible. Subsequently, thesacrificial layers 640 are removed by wet and/or dry etching operationsand thus, the EUV reflective structure as shown in FIG. 19 is obtained.

In some embodiments, when the sacrificial layer is silicon oxide, a wetetching operation using HF or BHF can be used to selectively remove thesacrificial layer 640.

FIGS. 20A-20D show another operation to remove the sacrificial layer 640according to an embodiment of the present disclosure. In someembodiments, the stacked structure shown in FIG. 18 is turned 90 degreesas shown in FIG. 20A. Then, an etching operation, such as a dry etchingoperation 680, is performed from one side of the stacked structure.Then, the stacked structure is rotated 90 degrees as shown in FIG. 20B,and then the etching operation 680 is performed from the next side ofthe stacked structure as shown in FIG. 20C. The etching operations arerepeated to fully remove the sacrificial layer 640 as shown in FIG. 20D.

FIG. 21 shows another operation to remove the sacrificial layer 640according to an embodiment of the present disclosure. In someembodiments, the stacked structure shown in FIG. 19 is placed on arotating table and an etching operation is performed from the side ofthe rotating stacked structure as shown in FIG. 21.

FIG. 22 shows another operation to remove the sacrificial layer 640according to an embodiment of the present disclosure. In someembodiments, an outlet hole is formed on at least the uppermost Molayer, to remove the etching residuals from inside to outside of thestacked structure as shown in FIG. 22.

FIGS. 23-26 shows views of various stages of a sequential manufacturingoperation for an EUV reflective structure according to other embodimentsof the present disclosure. Materials, configuration, dimensions,processes and/or method explained with respect to the above embodimentscan be applied to the following embodiments, and the detaileddescription thereof may be omitted.

As shown in FIG. 23, when the Si layer 620 is patterned, the etching isstopped not to expose the surface of the substrate 610. Then, thesacrificial layer 640 is formed, and a CMP operation is performed asshown in FIG. 24. The Mo layer 650 is then formed on the Si pattern 625and the sacrificial layer 640 as shown in FIG. 25.

In other embodiments, as shown in FIG. 26, an additional Si layer 625 isformed before the Mo layer 650 is formed.

It will be understood that not all advantages have been necessarilydiscussed herein, no particular advantage is required for allembodiments or examples, and other embodiments or examples may offerdifferent advantages.

In the present embodiments, the reflectivity of an EUV reflectivestructure can be enhanced by including nano cavities in the Si layers ofthe Si/Mo multiple reflective layer.

In accordance with one aspect of the present disclosure, an EUVreflective structure includes a substrate and multiple pairs of a Silayer and a Mo layer. The Si layer includes a plurality of cavities. Inone or more of the foregoing or following embodiments, each of theplurality of cavities has a polygonal pillar shape. In one or more ofthe foregoing or following embodiments, each of the plurality ofcavities has a cubic shape. In one or more of the foregoing or followingembodiments, each of the plurality of cavities has a cylindrical pillarshape. In one or more of the foregoing or following embodiments, theplurality of cavities are arranged in a matrix in the Si layer. In oneor more of the foregoing or following embodiments, the plurality ofcavities are arranged in the matrix with constant pitches in twodirections. In one or more of the foregoing or following embodiments,each of the plurality of cavities contains a gas. In one or more of theforegoing or following embodiments, each of the plurality of cavities isunder an atmospheric pressure. In one or more of the foregoing orfollowing embodiments, each of the plurality of cavities is filled witha dielectric material. In one or more of the foregoing or followingembodiments, a thickness of each of the multiple pairs is in a rangefrom 6.5 nm to 7.5 nm. In one or more of the foregoing or followingembodiments, a thickness Ts of the Si layer and a thickness Tm of the Molayer satisfy Ts:Tm=5:5 to 7:3. In one or more of the foregoing orfollowing embodiments, no cavity is disposed in the Mo layer. In one ormore of the foregoing or following embodiments, each of the plurality ofcavities is fully embedded in the Si layer. In one or more of theforegoing or following embodiments, one face of each of the plurality ofcavities is covered by the Mo layer. In one or more of the foregoing orfollowing embodiments, each of the plurality of cavities has an area inplan view at least 1 nm². In one or more of the foregoing or followingembodiments, the plurality of cavities have a same thickness as the Silayer. In one or more of the foregoing or following embodiments, the Silayer comprises a plurality of pillars. In one or more of the foregoingor following embodiments, the plurality of cavities are arranged in amatrix with a pitch p1 in one direction and a pitch p2 in anotherdirection crossing the one direction, and the Si layer has a thicknesst, and a volume Vn of each of the plurality of cavities satisfies0.3≤Vn/(t×p1×p2)≤0.9. In one or more of the foregoing or followingembodiments, each of the plurality of cavities has a volume in a rangefrom 1 nm³ to 1,000,000 nm³. In one or more of the foregoing orfollowing embodiments, a total number of the multiple pairs is at least40. In one or more of the foregoing or following embodiments, the EUVreflective structure further includes a cap layer over the multiplepairs. In one or more of the foregoing or following embodiments, the caplayer includes a material that protects the multiple pairs fromoxidation. In one or more of the foregoing or following embodiments, thecap layer includes an EUV absorber having a circuit pattern. In one ormore of the foregoing or following embodiments, the cap layer includes adiffractive grating. In one or more of the foregoing or followingembodiments, the EUV reflector has a curved shape forming a collectormirror.

In accordance with another aspect of the present disclosure, an EUVcollector mirror includes a substrate having a curved upper surface andmultiple pairs of a Si layer and a Mo layer disposed over the curvedupper surface. The Si layer includes a plurality of cavities.

In accordance with another aspect of the present disclosure, an EUVphoto mask includes a substrate, a multilayer structure disposed overthe substrate, the multilayer structure including multiple pairs of a Silayer and a Mo layer, and an absorption layer disposed over themultilayer structure. The Si layer includes a plurality of cavities.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. An extreme ultra violet (EUV) reflectivestructure, comprising: a substrate; and multiple pairs of a Si layer anda Mo layer, wherein the Si layer includes a plurality of cavities. 2.The EUV reflective structure of claim 1, wherein each of the pluralityof cavities has a polygonal pillar shape.
 3. The EUV reflectivestructure of claim 1, wherein each of the plurality of cavities has acubic shape.
 4. The EUV reflective structure of claim 1, wherein each ofthe plurality of cavities has a cylindrical pillar shape.
 5. The EUVreflective structure of claim 1, wherein the plurality of cavities arearranged in a matrix in the Si layer.
 6. The EUV reflective structure ofclaim 5, wherein the plurality of cavities are arranged in the matrixwith constant pitches in two directions.
 7. The EUV reflective structureof claim 1, wherein each of the plurality of cavities has a pressurelower than 1 Pa.
 8. The EUV reflective structure of claim 1, whereineach of the plurality of cavities is filled with a dielectric material.9. The EUV reflective structure of claim 1, wherein a thickness of eachof the multiple pairs is in a range from 6.5 nm to 7.5 nm.
 10. The EUVreflective structure of claim 9, wherein a thickness Ts of the Si layerand a thickness Tm of the Mo layer satisfy Ts:Tm=5:5 to 7:3.
 11. The EUVreflective structure of claim 1, wherein no cavity is disposed in the Molayer.
 12. The EUV reflective structure of claim 1, wherein each of theplurality of cavities is fully embedded in the Si layer.
 13. The EUVreflective structure of claim 1, wherein one face of each of theplurality of cavities is covered by the Mo layer.
 14. The EUV reflectivestructure of claim 1, wherein each of the plurality of cavities has anarea in plan view at least 1 nm².
 15. The EUV reflective structure ofclaim 1, wherein the plurality of cavities have a same thickness as theSi layer.
 16. The EUV reflective structure of claim 15, wherein the Silayer comprises a plurality of pillars.
 17. The EUV reflective structureof claim 1, wherein: the plurality of cavities are arranged in a matrixwith a pitch p1 in one direction and a pitch p2 in another directioncrossing the one direction, and the Si layer has a thickness t, and avolume Vn of each of the plurality of cavities satisfies0.3≤Vn/(t×p1×p2)≤0.9.
 18. The EUV reflective structure of claim 1,wherein each of the plurality of cavities has a volume in a range from 1nm³ to 1,000,000 nm³.
 19. An EUV collector mirror, comprising asubstrate having a curved upper surface; and multiple pairs of a Silayer and a Mo layer disposed over the curved upper surface, wherein theSi layer includes a plurality of cavities.
 20. An EUV photo maskcomprising: a substrate; a multilayer structure disposed over thesubstrate, the multilayer structure including multiple pairs of a Silayer and a Mo layer; and an absorption layer disposed over themultilayer structure, wherein the Si layer includes a plurality ofcavities.